1. Field of the Invention
This invention generally relates to an information recording and reproducing technology, such as an optical recording and reproducing system, and, in particular, to a system and method for reconstructing an information signal from the data recorded on a recording medium, such as an optical disk.
2. Description of the Prior Art
An optical disk system is well known in the art. In an optical disk system, data is recorded along an information track of an optical recording medium in the form of alternative positive and negative binary marks limited in length during a recording mode of operation. On the other hand, during a reproducing mode of operation, the information track of the optical recording medium is optically scanned by a light beam and the light intensity of the light reflecting from the optical recording medium or of the light transmitted through the optical recording medium is detected by a photodetector, whereby the detected light intensity is used to reconstruct an information signal.
Referring to FIG. 3a, there is schematically shown an example of a series of marks formed on an optical recording medium, such as an optical disk. As shown, a series of marks is arranged along an information track 10 which is typically defined on the surface of an optical disk spirally or circularly. The marks formed along the information track 10 include pits 12 and spaces 14 which are arranged alternatively. Each of the pits 12 is typically defined as a recess formed on the surface of an optical disk; on the other hand, each of the spaces 14 is defined as a spacing between the two adjacent pits 12. Typically, the length of each of the pit 12 and space 14 is limited to a predetermined value depending on a particular mode of modulation used, so that the pits 12 and spaces 14 each have a predetermined unit length in the direction along the information track 10. In FIG. 3a, there are shown pits 12 having a predetermined unit length and also a pit 12 having twice of the unit length. Similarly, there are also shown spaces 14 having a predetermined unit length and also a space 14 having twice of the unit length.
FIGS. 3b through 3d each graphically shows a detection signal which is obtained by scanning the marks comprised of pits and spaces formed on an optical system by scanning with a light beam and detecting the intensity or level of the light either reflecting from or transmitted through the optical disk. FIG. 3b shows a detection signal obtained from an optical disk on which pits and spaces are properly formed. In this case, if the average level of the detection signal is taken to be 0, then the length of pit T.sub.p defined between two adjacent zero-crossing points and the length of space T.sub.s defined between two adjacent zero-crossing points are both equal to T, and, thus, there is no discrepancy (so-called asymmetry) in length between the pits and spaces defined on the optical disk.
Incidentally, when the marks are formed on an optical recording medium, use is typically made of a laser beam. In this case, however, because of various reasons, such as fluctuations in intensity of a laser beam during a recording mode of operation, deformation, such as warping, of an optical recording medium, focus distortions, tolerances in the thickness of a substrate of an optical recording medium, aging of a recording medium, and stains of an optical system and an optical recording medium, there can be difference in recorded conditions as much as twice the power of a laser beam for use in recording. If the recording condition is above a proper recording condition, there is obtained a detection signal as shown in FIG. 3c, whereby T.sub.p and T.sub.s are not equal to T and T.sub.p is smaller than T.sub.s. On the other hand, if the recording condition is below the proper recording condition, there is obtained a detection signal as shown in FIG. 3d, whereby T.sub.p and T.sub.s are not equal to T and T.sub.p is larger than T.sub.s.
As described above, the occurrence of deviations of the actual recording condition from the proper recording condition during a recording mode of operation, in which data is recorded onto an optical recording medium, is unavoidable, and the occurrence of such deviations causes T.sub.p to be unequal to T.sub.s, thereby bringing about the before-mentioned problem of asymmetry. In particular, in the case of high density recording, such as 25,400 bits or more per inch, the shifting of bit position due to asymmetry, i.e., a deviation of bit timing from the intended timing, becomes larger, thereby presenting a practically grave problem.
As disclosed in the U.S. Pat. No. 4,142,208, which is hereby incorporated by reference, it has been proposed to maintain the duty of the output of a binary signal at 50% by detecting the duty ratio after converting a detection signal into a binary signal and feeding back a difference signal from the duty ratio of 50% to the input of a binary converting circuit. In this case, however, there still remains difficulty in adjustment of the feed-back circuit which is rather unstable and tends to oscillate. And yet, if the feed-back circuit is stabilized, then the advantages of providing such a feed-back circuit are significantly reduced.